with that utah doctor on trial for murdering his wife. Prosecutors say that he pressured his wife to get plastic surgery and killed her with a lethal dose of postop drugs. Reporter: Martin macneill appeared stoic at the beginning of his murder trial. During opening statements, prosecutors wasted no time digging into what they believe was his motive. This case starts a year and a half before, when the defendant met this person, gypsy willis. The two of them met online. Eventually, the relationship became romantic and sexual. Reporter: Around the same TIME, PROSECUTORS say MacNeill started behaving erratically. And convincing his wife, michelle, to get a face-lift. The couple argued about this. And the defendant announced that the surgery was paid for and that she was going to do it. Reporter: Prosecutors say MacNEILL OVERMEDICATED HIS WIFE With a deadly dose of pills while she was recovering from that surgery. MacNEILL SHOWED LITTLE EMOTION As his frantic 911 call was replayed. He did shake his head multiple times when the prosecution referred to testimony from inmates. Defendant told another inmate that he had simply given michelle too much medication and caused her death. Reporter: Then, it was the defen defense's turn. You may say he is a total jerk. But it's critical that during this trial you set aside your emotions and you evaluate this case based upon the facts. Reporter: She said the real culprit behind michelle's death was heart disease. The medical examiners in this case agree that michelle's heart disease was either the cause or the contributing cause to her death. Reporter: The defense also questioned the credibility of one of the prosecution's star witnesses. MacNEILL'S DAUGHTER, ALEXIS, WHO Has long believed that her father is guilty. The evidence will show that alexis has made multiple inconsistent and inaccurate statements about the events surrounding her mother's passing. Reporter: Jurors also heard from two of the prosecution's witnesses. Both doctors. Dr. Scott thompson performed the surgery on michelle. And dr. Von wells examined her before her face-lift and found high blood pressure. Attorneys and witnesses piecing together a puzzle for jurors to solve. For "good morning america," aditi roy, abc news, provo, utah. Let's get analysis from dan abrams. You saw the strategy there. Prosecutors going hard and loaded. The defense saying, the medical examiners didn't say it was murder. That's the heart of the defense. You can't prove that this was a hurd. And in fact, the medical examiner who looked at this said it was natural causes. That is the biggest hurdle for prosecutors to overcome. But you heard there, aditi's piece, the defense going after the daughters. And that's a really important point in this case. Smart, though? They have to, to some degree. The daughters in particular, alexis, there's four biological daughters, is going after her father. And has been. She is the reason that this case went to trial because the daughters didn't give up on this. Doors were closed on them again and again. And they kept saying, dad did it. Dad did it. And now, she's going to be a crucial witness for the prosecution. So, the defense has to go after her. A couple of points from the doctors that performed the surgery. He didn't want to give certain DRUGS, DR. MacNEILL WANTED. The doctor says, I don't usually prescribe these kinds of drugs. BUT DR. MacNEILL BEING A DOCTOR, I deferred to him and was willing to prescribe the more aggressive drugs. NUMBER TWO, IT WAS DR. MacNEILL, More than his wife, that seemed obsessed about getting the face-lift. That's two key points coming from the doctors that testified so far. And the doctors will say she was taking too much of the medication, as well. That's right. At the behest of DR. MacNEILL.

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Martin MacNeill Trial: Tough Case for Prosecution?

Dan Abrams and Nancy Grace discuss the trial of the doctor accused of wife's killing.

Trial to Begin for Doctor Accused of Killing Wife

Physician and former Sunday school teacher accused of giving his wife a deadly dose of painkillers.